Countermeasures have been used almost since the inception of radar for lessening the effectiveness of target detection. Early countermeasures included chaff, which consisted of strips of conductive metal foil dropped in an area, to simulate a target. The jamming technique relies on use of an active transmitter which transmits a signal which overwhelms the radar receiver, or which deceives the radar system processing into perceiving targets at locations at which they do not exist. A traditional way for the radar operator to overcome jamming is to adjust his antenna so that a less responsive portion of the antenna beam pattern, or preferably a null is directed toward the jammer. When a beam null is directed toward the jammer, the jammer signal at the receiver is minimized, and the jammer may be rendered ineffective. With simple antennas, it is difficult to generate plural nulls which would make it possible to reject the signals from a number of simultaneously operating jammers.
With the advent of modern array antenna techniques, it became possible to generate multiple antenna beams simultaneously or in a short time sequence. This gave rise to the ability to track multiple targets either simultaneously or quasi-simultaneously, using computer-controlled antenna beam controllers and signal processing. Computer control also gave rise to autonomous jammer nulling techniques, in which the array antenna could be controlled by an autonomous system dedicated to the reduction of the jamming signal from a single jammer, without excessively deforming the main or target detection beam. U. S. Pat. No. 5,600,326, issued Feb. 4, 1997 in the name of Yu et al. describes one such jammer nulling technique. Other techniques include those described by S. P. Applebaum in the article "Adaptive Arrays," IEEE Transactions on Antennas and Propagation, vol. AP-24, No.5, September 1976, pp 585-598, and in an article by Widrow et al. entitled "Adaptive Antenna Systems," Proceedings of the IEEE, vol 55, December 1967, pp 2143-2159. With such autonomous jamming reduction capability, the radar system operation is essentially freed from the need to devote radar processing resources to jammer signal reduction.
Improved methods and arrangements are desired for determining the direction of a jammer.